The present invention relates to a hydraulic system. The invention also relates to a working machine comprising the hydraulic system.
A working machine in the form of a wheel loader has a plurality of different work functions which are controlled hydraulically, such as lifting and tilting of an implement and steering (frame steering) of the working machine. The control of the respective work function is performed via hydraulic actuators; such as linear motors in the form of hydraulic cylinders.
Below, the invention will be described in connection with the operation of a wheel loader. This is a preferred, but by no means limiting application of the invention. The invention can also be utilized for other types of working machines or working vehicles having hydraulic work functions. It could for example also be an articulated hauler, a backhoe loader, an excavator, or an agricultural machine such as a tractor.
The present hydraulic systems are preferably of a load sensing type (L S systems). This means that the pump supplying the system with hydraulic oil senses the pressure (via a L S signal) from the actuated hydraulic cylinder (hydraulic cylinders). The pump then sets a pressure which is slightly higher than the pressure in the hydraulic cylinder. Thereby, a flow of hydraulic oil out to the hydraulic cylinder is obtained. A control valve (also called a manoeuvre valve) is placed between the pump and the hydraulic cylinder.
The magnitude of the flow to the hydraulic cylinder depends on how much the actuated control valve is modulated open.
In the present hydraulic systems of load sensing type, energy which could be recovered is lost. Some examples of energy losses which may arise will be described below.
An operation mode where an energy loss may arise is when lowering a work implement, such as a bucket or a container, wherein the intrinsic weight (and in some cases load) of the work implement drives the piston in the hydraulic cylinder. Here, a pressure drop usually occurs across the control valve, since the returned hydraulic oil is drained to tank, which in its turn results in an energy loss (heat). Another operation mode where an energy loss may arise is with so-called back up pressure. When the steering of the working machine is used, the returned hydraulic oil is pressurized with a back pressure of the mapitude of approx. 10-40 bar with the purpose of obtaining a stable steering without jerks. This back up pressure, in its turn, leads to energy losses. Another operation mode where energy loss may arise is during so called parallel operation of different work functions. In general, a single common pump is used for a plurality of work functions. These work functions may, however, require different pressures, meaning that the pump has to be adjusted according to the highest required pressure. This means that, during parallel operation of two work functions having different pressure requirements, the pressure has to be reduced for the work function requiring the lowest pressure. The pressure drop which arises across the control valve for the work function requiring the lowest pressure results in an energy loss,
It is desirable to produce a hydraulic system of the kind defined by way of introduction, which system creates conditions for a more efficient operation of the hydraulic system, and/or of a working machine provided with such a hydraulic system, with respect to energy consumption.
Instead of having losses arising due to a pressure drop across the control valve unit (as described above), energy can be recovered with the recovery unit. Since the pressure limiting means comprises a pilot-operated valve adapted to set a maximum allowable pressure at the return port of the control valve unit, which pressure is variable by controlling the pilot-operated valve by means of a control unit, an upper limit for the amount of energy desired to be recovered from the work function can be selected.
The set maximum pressure also determines the smallest possible pressure drop across the control valve unit. A return flow of hydraulic oil from the work function will flow through the recovery unit and energy will be recovered as long as the recovery unit does not produce a higher back pressure than the set maximum allowable pressure. The invention creates conditions for controlling the energy recovery in a variable way depending on the actual operating mode. Within the range of the maximum allowable pressure, the pressure drop across the control valve unit will be determined by the resistance from the recovery unit. In many cases, the pressure drop across the control valve unit is preferably as small as possible in order to maximize the energy recovery, but sufficiently large to achieve the modulation of the requested return flow of hydraulic fluid. The recovery unit can, for example, be a hydraulic machine functioning as a hydraulic motor when recovering energy. The recovered energy can go directly to a consumer or be stored in a suitable manner.
Even though the recovery unit is adapted to the actual hydraulic system, in some cases it could happen that all energy which potentially can be recovered at a certain point in time cannot be stored or consumed instantaneously. In such a case, a certain amount of energy can still be emitted in a conventional manner in the form of heat resulting from a pressure drop across the control valve unit and/or the pilot-operated valve. In case only a limited energy recovery or no energy recovery at all is desirable in a certain situation, the pilot-operated valve can be controlled so that the maximum allowable pressure at the return port is low (relative to the pressure of the work function in question) or, in the latter case, so that the maximum allowable pressure at the return port is essentially negligible. If, on the other hand, it is desired to recover as much energy as possible, the pilot-operated valve can be controlled so that the maximum allowable pressure is high (of the same mapitude as, or higher than the pressure of the work function in question). In that case the recovery unit is controlled so that the desired recovery is obtained, at the same time as it is ensured that the pressure drop across the control valve unit is sufficiently large to achieve the modulation of the requested return flow of hydraulic fluid.
It should be pointed out that the expression “return port of the control valve unit” can include a separate outlet from a valve (if the control valve unit comprises a valve with an outlet or a return port), as well as a common connection point for two or more outlets of one or several valves (if the control valve unit, for example, comprises two control valves). The primary thing is that, by means of the pilot-operated valve, the maximum allowable pressure of the return flow downstream of the control valve unit can be controlled to the desired level.
The pilot-operated valve is preferably connected to the return port and connected in parallel with the recovery unit, which means that hydraulic fluid can be directed to the recovery unit and/or via the pilot-operated valve past the recovery unit and further for example to tank.
The pilot-operated valve is preferably electrically controllable and, furthermore, the maximum allowable pressure at the return port of the control valve unit is preferably continuously variable by means of the pilot-operated valve. By means of a control unit and a suitable software, the pressure at the return port of the control valve unit can be adjusted and adapted to the actual operating situation in order to optimize the energy recovery. An electrically controllable valve creates conditions for controlling the energy recovery in an accurate manner.
As described above, the pressure limiting means comprises a pilot-operated valve. This valve can, for example, be a pressure limiting valve or a proportional directional valve which, by means of a control unit and pressure sensors, functions as a pressure limiting valve.
The expression “pilot-operated” valve refers to a valve, the reference value of which (pressure or flow) is determined by an external signal (electric or hydraulic), preferably from a control unit. This in contrast to a valve which is direct acting, i.e. a valve which is adapted to respond to a specific condition (usually a pressure) in the system and which, accordingly, has a setting which is fixed in relation to the prevailing condition. For example, such a direct acting valve can have a given pressure level which is determined by a preloaded spring.
The invention has particular advantages in case the hydraulic fluid delivered by the main pump has a pressure which exceeds the pressure required for a certain work function in a given operating situation. Such an excessive pressure could be the result of the system having a pump operating at a constant pressure level, but it is more common when using one and the same pump for two or more work functions that different pressures are required for the work functions and the pump pressure then has to be adapted to the work function requiring the highest pressure. If, for example, hydraulic cylinders for lifting and steering are used simultaneously, the lift function may require a pressure of 200 bar and the steering may require 50 bar. With the system according to the invention, the control valve unit will not have to be used to reduce the pressure to the steering to approx. 50 bar with associated energy losses. Instead, the recovery unit can boost the pressure of the return flow from the steering with 150 bar in order to obtain the required pressure difference of 50 bar (200−150=50). This means that the pump pressure of 200 bar can be used both for the lift function and for the steering. Since the pressure drop instead occurs across the recovery unit, energy from the steering will be recovered and the recovered energy (with the exception of component-related losses) will be proportional to the product of the volume of hydraulic fluid passing the recovery unit multiplied by the pressure drop across the recovery unit.
As indicated above, the invention can advantageously be applied to a hydraulic system comprising a plurality of work functions and, according to one embodiment of the invention, the hydraulic system comprises a plurality of work functions with associated respective control valve unit (which control valve units, however, in their turn can be integrated into a common overall fluid control means for two or more work functions), and one said pilot-operated valve is provided for the respective work function. This gives a hydraulic system which creates conditions for recovering energy from any one of plurality of work functions in an efficient manner. It is possible to control which work function energy should be recovered from and to what extent energy should be recovered. This results in a very flexible system which enabled the total energy consumption of a working machine to be reduced considerably. The recovery unit is preferably arranged in parallel with all said pilot-operated valves, although it would also be possible to use a plurality of recovery units provided for different work functions. It shall be pointed out that the different variants of the hydraulic system described in connection with a work function of course also can be applied to two or more work functions.
By setting a pressure level for a first work function, with the pilot-operated valve, which enables a certain pressurization of the hydraulic machine, at the same time as the pilot-operated valves for the other work functions are set to a lower pressure level, hydraulic fluid from the first work function will be directed to the hydraulic machine, while hydraulic fluid from other work functions instead will be directed to tank via the respective pilot-operated valve.
Furthermore, conditions are created for enabling the part of the hydraulic system related to energy recovery to be designed as a separate unit, which can be connected to a given hydraulic system. Such a separate unit can be connected to the return side of one or several work functions in different types of hydraulic systems. Accordingly, an energy recovery system can be built as a separate unit and be offered as an option to a standard system. In the following text, the expression “energy recovery system” will be used for the part of the hydraulic system capable of constituting such a separate unit which can be connected to a base system in a simple manner.
The hydraulic system preferably comprises a pump, hereinafter also called a main pump or supply pump, for providing hydraulic fluid to said at least one work function. Such a pump can be adapted to supply one or several work functions with hydraulic fluid. According to one embodiment of the invention, the hydraulic system comprises a means for returning energy, recovered from a return flow from the work function, to the pressure side of the pump. This offers a possibility to recover energy which is then used to assist the main pump in supplying one or several work functions. This in its turn creates conditions for solving, or at least reducing, the problem of providing enough energy to drive the hydraulic system and the driveline at low engine speeds in a working machine. The main pump in a hydraulic system of the kind in question is namely usually mechanically connected to the engine of the working machine, such as a diesel motor, which is used to drive both the hydraulic system and the driveline for propelling the working machine. The speed of the main pump will thus become dependent on the speed of the diesel engine. The speed of the diesel engine, in its turn, depends on the desired propulsion speed of the working machine and the torque determined by the actual operation mode.
According to another embodiment of the invention, the recovery unit comprises a first hydraulic machine and a second hydraulic machine, and the first and second hydraulic machine are mechanically interconnected, and the first hydraulic machine is adapted to be driven by a flow of hydraulic fluid and the second hydraulic machine is adapted to pump hydraulic fluid by being driven by the first hydraulic machine. The first hydraulic machine is preferably connected to the return port to be driven by a return flow from the work function and the second hydraulic machine is adapted to pump hydraulic fluid from, for example, a tank to the pressure side of the main pump and/or to an accumulator and from the accumulator further to the pressure side (or suction side) of the pump. When using an accumulator, hydraulic fluid can be provided directly from the accumulator to the main g pump, or via the recovery unit, in that the accumulator supplies the first hydraulic machine and the second hydraulic machine pumps hydraulic fluid to the pressure side (or suction side) of the main pump.
Accordingly, with a suitable recovery unit, hydraulic fluid can be provided for supplying a work function in a way which is independent of the engine speed of the working machine. In many situations this flow, together with the flow generated independently by the main pump, provides a sufficient flow to the work functions, also if the diesel engine is operating at a low speed causing the capacity of the main pump to be reduced. Thus, in other words, the recovered energy stored or used instantaneously can be used for supporting the diesel engine.
In case a more simple recovery unit is used, for example in the form of a simple hydraulic machine, energy can also be recovered and returned to the pressure side of the main pump. If there is a sufficiently high pressure on the return flow, at least part of the flow could be returned directly to the pressure side of the main pump. Should the pressure be too low, the hydraulic machine could be used as a pump to increase the pressure so that the return flow can be returned and, in some cases, if the pressure of the return flow exceeds the pressure on the pressure side of the main pump by some margin, part of the energy could first be recovered in the hydraulic machine, and thereafter the return flow could be returned to the pressure side of the main pump.
According to another embodiment of the invention, the hydraulic system comprises a means for returning energy recovered from the work function to the suction side of the pump. There is of course a possibility to, at least to a certain extent, obtain the above-mentioned advantages with supporting the main pump also by instead providing hydraulic fluid to the suction side of the main pump. For instance, in case the hydraulic fluid pressure of the return flow is not sufficiently high to enable returning of hydraulic fluid to the pressure side, the energy can be utilized by returning it to the suction side of the main pump, since the main pump does not have to increase the pressure of this hydraulic fluid as much as if it instead had drawn hydraulic fluid from a tank. Thus, the variants described above with respect to return of energy (or in other words hydraulic fluid) to the pressure side of the main pump can also be applied with respect to return to the suction side of the main pump. In case of an excess of return flow, a certain amount can be directed to tank and/or be intermediately stored in an accumulator for successive return to the main pump.
According to a further embodiment of the invention, a pump, preferably the main pump for supplying the work functions, is drivable by a driveline of a working machine and adapted to brake the driveline during deceleration of the working machine, and the system further comprises a hydraulic control means for controlling a flow of hydraulic fluid, from the pressure side of the pump to the recovery unit, for recovering energy during deceleration of the working machine. Thereby, the recovery unit can also be used to decelerate the working machine, at the same time as deceleration energy is recovered during deceleration of the working machine.
According to a further embodiment of the invention, the recovery unit is adapted to dampen a relative movement caused by an external disturbance, at least in one direction, between a work implement and a machine body of the working machine, which work implement is moveable relative to the machine body by means of said work function. Preferably, the hydraulic system comprises a sensor for determining a reference position for the work implement relative to the machine body. Thereby, the recovery unit can recover energy at the same time as it is part of a suspension system for, for example, the lift arm of a wheel loader. With a suitable control of the recovery unit and the rest of the hydraulic system, a damped suspension system for a work implement can be achieved, at the same time as energy can be recovered with the recovery unit.
By means of the method according to the invention, energy can recovered with the recovery unit in a corresponding way as has been described above with respect to the hydraulic system. Since the maximum allowable pressure at the return port is controlled with a pressure limiting means through receiving signals, which preferably are electric, from a control unit, an upper limit for the amount of energy that is desired to be recovered from the work function can be selected.
The set maximum pressure also determines the smallest possible pressure drop across the control valve unit. A return flow of hydraulic oil from the work function will flow through the recovery unit and energy will be recovered as long as the recovery unit does not generate a higher back pressure than the set maximum allowable pressure. The invention creates conditions for controlling the energy recovery in a variable way depending on the actual operating mode. Within the range of the maximum allowable pressure, the pressure drop across the control valve unit will be determined by the resistance from the recovery unit. In many cases, the pressure drop across the control valve unit is preferably as small as possible in order to maximize the energy recovery, but sufficiently large to achieve the modulation of the requested return flow of hydraulic fluid. The recovery unit can, for example, be a hydraulic machine functioning as a hydraulic motor when recovering energy. The recovered energy can go directly to a consumer or be stored in a suitable manner.
The invention furthermore relates to a working machine provided with the hydraulic system according to the invention.
Further advantages and advantageous features of the invention are evident from the detailed description below and the following claims.